An advanced hydrogen-based fuel cell power-train system for the automotive industry must meet a number of demanding requirements to become commercially attractive. These include improvements in fuel economy, power efficiency and durability in order to provide a vehicle range equaling that of an internal combustion engine—all of which have to be achieved while trying to meet cost targets. Automotive data indicate that dissolution of the fuel cell catalyst material, in particular Pt dissolution, is a significant degradation mode that currently limits the lifetime of the membrane electrode assemblies (MEA) of a fuel cell. Although, in the following the invention is disclosed focusing on Pt, the invention is not limited to Pt as a catalyst material. Other catalyst material may also be employed, for example alloys containing Pt, e.g. PtCo, or not.
Polymer electrolyte membrane (PEM) fuel cells are electrochemical devices that take a flow of hydrogen and air and convert them, electrochemically, into electrical power and heat. PEM fuel cells can be used in a wide range of applications, including stationary power-generating units, back-up power systems and in transportation. There are currently a number of issues that need to be addressed before large-scale commercialization of hydrogen fuel cell technology can happen.
Key issues regarding the main technical challenges to automotive fuel cells are directly associated with MEA components. They include, for example:                The cost of the membrane electrode assembly (MEA) needs to be reduced. The cost requirement is one of the primary drivers for the hydrogen fuel cell (HFC) automotive technology. A significant part of the costs is allotted to the expensive Pt-catalyst material. Usually, a higher amount of Pt is used as is actually needed to compensate for the catalyst degradation during operation of a fuel cell.        The MEA durability needs to be improved considerably to address to address catalyst degradation. More specifically, more than 4000 hours of a driving cycle are required. There also should be minimal loss of power caused by the following degradation phenomena in the MEA: loss of power because of Pt-catalyst agglomeration and dissolution; carbon corrosion; and other degradation mechanisms.        The mass activity of the oxygen reduction reaction (ORR) catalyst needs to be increased by at least a factor of four. The current assumption is that future HFC automotive systems will have less than 0.3 mg/cm2 of the total catalyst loading, in particular Pt loading, in their MEAs.        The gas permeability of the PEM (to hydrogen, nitrogen and oxygen) needs to be reduced.        
Though a fuel cell stack has no moving parts—which is a benefit for overall durability and reliability—a fuel cell consists of a number of materials that perform a variety of functions. Durability requirements for automotive fuel cells are particularly aggressive because of the wide range of temperatures, relative humidity (RH) and pH conditions that the cell must operate under for extended periods. Typical MEA stressors include:                Start-up and shut-down transients;        load/voltage cycles;        temperature cycles (less than 0° C. to in excess of 95° C.);        RH cycling; and        cell reversal.        
One fuel cell material of particular interest is the cathode catalyst layer, which is responsible for facilitating the oxygen reduction reaction (ORR), and is also required to transport oxygen and water, conduct heat, protons and electrons. Any deficiency or degradation of one of these functions impacts the entire cell and system.
Much of the variability in automotive conditions stems from local driving conditions, such as different seasons and weather. In addition to such external factors, driver behavior has a significant impact on how the cell operates and thus what stressors may be applied to the fuel cell. A typical automotive duty cycle consists of many load transients, holds, periods of idling and start-up/shut-down sequences that are related to driver behavior. Load/potential cycling in a fuel cell vehicle (FCV) contributes to various operating conditions that may accelerate degradation of MEAs. It is well known that potential cycling is detrimental to the cathode Pt-catalyst especially when higher potentials (>1.0 V) are reached. Moreover, it is well-known that load cycling/potential cycling along with start/stop cycles primarily result in catalyst and catalyst support degradation through the dissolution and redeposition of the Pt. Understanding the relationship between operating conditions and degradation mechanisms enables more robust fuel cells to be developed.
Analysis of MEAs subjected to testing based on a user load profile has shown signs of Pt dissolution. As such, a great deal of research effort is going into gaining an understanding of the Pt dissolution mechanism. The basic mechanisms of Pt dissolution during a voltage cycle have been studied by many research groups. For example, it has been shown that Pt can dissolve at potentials greater than 1 V. At these high potentials, Pt dissolves electrochemically:Pt→Ptn++n e−n=2, 4, 6
The cathode potential in an automotive fuel cell typically ranges between approximately 0.6 V to 0.95 V during normal operation. However, excursions up to 1.5 V are possible during start-up and shut-down events. Even though high voltages encourage the formation of a Pt oxide layer which has been shown to slow the Pt dissolution rate, a voltage cycle that repeatedly forms and removes this layer, such as in a vehicle cycle, may have a very high dissolution rate.
Because of the costs and effort involved in preparing large stacks, and the long lifetimes that are necessary for fuel cell products, it is not feasible to perform research and development activities on this scale. Thus, to facilitate material screening and general research activities it is necessary to use small-scale fuel cells and accelerated stress tests (ASTs). Such protocols are also useful for targeting and understanding specific degradation mechanisms. An AST that is commonly used for measuring the stability fuel cell catalysts is voltage cycling. This stress test magnifies the dissolution and agglomeration process while attempting to mimic the voltage cycling that is seen in vehicles. Many voltage cycling ASTs cycle cell voltage between 0.6 V and 1.2 V while flowing humidified hydrogen on the anode and humidified air on the cathode, however there are many variations. Protocols in the literature vary by the voltage window, the voltage profile (square wave, triangle wave or sinusoidal) and input gasses, which results in different degradation rates. One of the protocols that simulates actual vehicle conditions draws a load using a load-bank to bring cell voltage to 0.6 V and then switches to a power supply to boost voltages to 1.2 V with air and hydrogen on the cathode and anode, respectively, in a square wave pattern.
Observations from using a voltage cycling AST include decreased performance; a decreasing electrochemical platinum surface area (EPSA); migration of Pt into the ionomer membrane, forming a Pt-band; and a Pt-depleted zone in the cathode catalyst layer. Application of the voltage cycling AST results in a significant performance loss. This is largely attributed to a decrease in the cathode EPSA that initially degrades quickly over the first 1000 cycles and then begins to plateau with increasing cycles. Depending on the type of catalyst used, this plateau may be non-zero. The reason why a non-zero plateau exists is still under investigation, however, it is likely because of a combination of the stabilisation of the Pt-particles as well as transport limitations brought about by an increasing distance between Pt-particles in the catalyst layer and the ionomer membrane.
A common observation from a voltage cycling AST is the migration of Pt into the electrolyte membrane. FIG. 1 compares transmission electron microscopy (TEM) images of a catalyst-coated membrane (CCM) at beginning of life (BOL) with the same type of membrane after voltage cycling. At the beginning of life (FIG. 1a) the electrolyte membrane is free of any visible contaminants. However, after voltage cycling (FIG. 1b) Pt can be seen in the membrane where it has formed a distinct (Pt) band approximately 3.8-4.2 μm from the cathode catalyst layer edge. Another observation is the presence of a Pt-depleted region in the catalyst layer at the catalyst-membrane interface. This can be seen in FIG. 1c, and indicates that the Pt-band originated from the cathode catalyst layer. Within the membrane the Pt may precipitate out in a variety of sizes. Analysis of Pt-particle morphology in the Pt-band may provide additional information on the mechanism of formation of the Pt-band. For example, evidence of the existence of the single and isolated Pt-particles in the membrane suggests that the growth of Pt started with the reduction of Pt-cations in the ionomer matrix. However, details of further dynamics of the growth of the Pt-particles into larger clusters are not yet clear. For more detail see S. Kundu, M. Cimenti, S. Lee, and D. Bessarabov, “Fingerprint of Automotive Fuel Cell Cathode Catalyst Degradation: Pt band in PEMs”, Membrane Technology 10 (2009) 7-10.
Thus, a MEA with improved stability against Pt dissolution and Pt-band formation needs to be developed for the industry to overcome the degradation problem.